The principal phenomenon under study is sex differentiation of the brain, both normal and sex hormone-reversed. A primary objective is study of the influence of hormones on qualitative aspects of transcription of RNA in particular regions of the brain. Toward this end, methods are being developed for 1) obtaining increased specific activity of radioactive labeling of brain RNA and 2) increased sensitivity of the DNA:RNA hybridization procedures for detecting qualitative features of brain RNA. We have made considerable progress in both of these technical areas and hope to be soon in a position to make relatively fine characterizations of RNA transcription in relatively small regions of the brain. These improved techniques should be generally useful to a wide group of other investigators interested in the molecular aspects of central nervous function. A second set of studies that we are just beginning is the localization of the center(s) for reproductive behavior in male frogs. With these centers localized and identified, we wish to determine their hormonal sensitivity and regulation, as well as the hormonal responsiveness of afferent nervous pathways to the (presumably hypothalamic) reproductive behavioral centers.